1. Field of the Invention
The present invention relates to an optical transmission system suitable for bidirectional transmission, and also to a terminal device applicable to this system.
2. Description of the Related Art
The international standard specifications of an ATM-PON (Asynchronous Transfer Mode-Passive Optical Network) system are defined in ITU-T: G983.1. This system includes a plurality of ONUs (Optical Network Units) as subscriber terminals, an OLT (Optical Line Terminal) as an office terminal device, and an optical star coupler for connecting the ONUs to the OLT so that bidirectional transmission is allowed between each ONU and the OLT.
In general, the transmitting power of each ONU is constant. Accordingly, the receiving power of the OLT is attenuated by losses in optical fiber transmission lines for connecting each ONU and the OLT, and the dynamic range in a receiving section of the OLT reflects a difference in the losses between the optical fiber transmission lines. Accordingly, a circuit in the receiving section of the OLT is required to instantaneously discriminate between “0” and “1” in a burst data sequence having a large dynamic range, so that the configuration of the circuit becomes complex. Further, in the case that there arises a requirement for expansion of a network, such as a requirement for relocation of a certain one of the ONUs from an original position to a far position or to a near position or a requirement for increase in the number of subscribers, it is required that the transmitting power of each ONU reaching the OLT falls within a receivable level range of the OLT, causing a difficulty of flexible expansion of the network.
As measures against such a problem, it is effective to detect a transmission line loss from the receiving power of each ONU, control the transmitting power of each ONU according to the result of this detection, and reduce the dynamic range in the receiving section of the OLT. For example, Japanese Patent Laid-open No. Hei 4-269023 discloses an optical transmission system having means provided in the OLT for outputting an optical signal at a substantially constant level, means provided in each ONU for detecting a transmission line loss from the level of a received optical signal, and means provided in each ONU for adjusting the level of an optical signal to be transmitted from each ONU according to the result of this detection of the transmission line loss so that the OLT can receive an optical signal at a substantially constant level. However, the purpose of reducing the dynamic range in the receiving section of the OLT in this prior art system is to facilitate the design of a circuit in the receiving section, and there is no mention about a method for increasing the dynamic range in a transmitting section, which becomes important in aiming at the extension of a network. Accordingly, the effect of the extension of a network by the prior art system is low.